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The thief ant Solenopsis tennesseensis is about the size of a poppy seed.

April Nobile/© (CC-BY-SA-3.0)

Thief ants steal—and eat—the young of other ants, decimating their populations

Even ants have to deal with pests. One of those is the pesky, poppy seedsize thief ant, which steals and eats the young of larger ants. A new study reveals such foraging exacts a heavy toll on other ant species, with damage so severe it can cascade up the entire food chain.

“It’s just staggering,” says Andrea Lucky, an ant systematist at the University of Florida, who was not involved with the work, but who now advises the author of the paper. “Thief ants are formidable predators.” 

Scores of species of thief ants populate the New World. Most live underground, tunneling into the nests of larger ants and stealing young from ants up to 24 times their size. They spray a powerful venom to keep the adults at bay.

Though abundant, thief ants have been ignored by most scientists because of their small size and underground existence. Indeed, “These subterranean communities have even been called ‘the final frontier for ant biodiversity,’” says Michael Kaspari, a community ecologist at the University of Oklahoma.

Leo Ohyama, then a masters student at the University of Central Florida, decided to see what happens when thief ants go missing. He set up 20 plots of 18 square meters—about the size of a double-wide volleyball court—in a state park known for its sandy soil. Then, each month for 14 months, he buried plastic tubes with insecticide-laced bait in 10 of the plots. A screened hole at the bottom of the tube kept out any ants bigger than a thief ant.

Overall, the insecticide treatment reduced the number of thief ants by 71%—and increased the number of the larger ants by 35%, Ohyama and his colleagues reported last month in Ecology.

The effects were particularly dramatic as the second year of the study reached May and June, when ant numbers are highest. Then, the team observed an 82% decrease in thief ants and a 65% increase in larger ants.

The thief ants appear to target some species more than others. When thief ant numbers declined, Pyramid ant (Dorymyrmex bureni) numbers almost doubled, while the nocturnal Nylanderia arenivaga ant had a 98% increase in workers. “This suggests that these predatory activities are highly complex and may have evolved over a long period of time,” Ohyama says, with some ants developing defenses against the thief ants.

Jean-Philippe Lessard, a community ecologist at Concorrida University, praises the effort put into this study. But he suggests the experiment needed to run longer to be sure the documented effects were long-lasting. Also, it would have been better to assess colony density, he says, not just the numbers of ants, which can vary quite a bit.

Although humans don’t usually need to worry about thief ants in their kitchen, they should care what these ants do to other ants, says Walter Tschinkel, an ant specialist at Florida State University. In general, ants turn over soil, making it easier for plants to grow; the insects are also are food for caterpillars, spiders, lizards, and other creatures that keep ecosystems functioning. Indeed, Ohyama says, they may be as important as other keystone predators, such as wolves in Yellowstone National Park. And, Lessard says,“Maybe they will keep in check other, more annoying ants in your garden or backyard.”